Corrosion resistant ornamental chromium plating

ABSTRACT

CHROMIUM PLATING WITH SUBSTANTIALLY IMPROVED RESISTANCE TO CORROSION AND FLAKING IS PRODUCED BY FIRST ELECTROPLATING THE SUBSTRATE WITH A HIGHLY STRESSED NICKEL LAYER SUCH THAT UPON ELECTROPLATING THE CHROMIUM THEREON MICRO-CRACKS FORM IN THE NICKEL AND CHROMIUM LAYERS. THE MICRO-CRACKING NICKEL LAYER IS DEPOSITED FROM A BATH CONTAINING NICKEL CHLORIDE, AN ORGANIC ACID AND THE NICKEL SALT THEREOF SELECTED FROM THE GROUP CONSISTING OF GLUCONIC, TARTARIC, FORMIC, MALIC, LACTIC, CITRIC AND SUCCINIC ACIDS. IN CERTAIN APPLICATIONS NICKEL SULFAMATE OR NICKEL FLUOBORATE MAY BE SUBSTITUTED FOR THE NICKEL CHLORIDE.

United States Patent Oifice 3,759,802 Patented Sept. 18, 1973 3,759,802 CORROSION-RESISTANT ORNAMENTAL CHROMIUM PLATING Gaetan de Coye do Castelet, Billaucourt, France, assignol' to Regie Nationale des Usines Renault, Billancourt, Hauts de Seine, France No Drawing. Continuation-impart of application Ser. No. a 153,828, June 16, 1971, which is a continuation of application Ser. No. 482,902, Aug. 26, 1965. This application Sept. 27, 1971, Ser. No. 184,251 Claims priority, application France, Mar. 31, 1965,

Int. c1. czsb 5/08, 5/50 US. Cl. 204-41 3 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-impart of my copending application Ser. No." 153,828, filed June 16, 1971, now abandoned, which is a streamlined continuation of my then copendingapplication Ser. No. 482,902, filed Aug. 26, 1965, now abandoned.

' This invention relates to various bath formulas for the electrodeposition of cracking nickel or nickel-alloy deposits prior to the electrodeposition of chromium.

In copending application Ser. No. 440,019, filed Mar. 15, 1965, now Pat. No. 3,388,049, there is described a method for producing such cracking nickel deposits from electroplating baths containing as basic components the following two ingredients: nickel chloride, nickel acetate.

The conditions for producing nickel plating having the desired properties as described in said application are illustrated by the following examples:

EXAMPLE 1 Principal protective nickel plating, for example bright nickel plating Rinsing operations Depositing of the cracking nickel under the following conditions:

Optimum Anhydrous nickel acetate 50 g.fl. to saturation-.. 100 g. l. Hydrated nickel chloride 100 g./l. to saturation 375 gJl. pH 2 to 6 5 to 5.5. Current density 1 to 15 A./dm. 10 Aldrin. Duration sec. to 2 min-.- 30 sec. Temperature Below 35 C to C.

Nora-Agitation optional (with compressed air, say).

Rinsings Chromium plating Rinsings Drying r EXAMPLE 2 Principal protective nickel plating (bright nickel plating, v for example) Rinsings .5 Depositing. ofcracking nickel, ,in the following solution:

Optimum Hydrated nickel chloride g./l. to saturation 400 gJl. Anhydrous ammonium acetate- 50 g./l. to saturation... 100 g./l. H 2 0 6 5 to 5.5.

l to 15 A.,|'dm 2 l0 A./dm 1 15 sec. to 2 min 30 sec. Temperature Below 35 C 20 to 25 0 N oTE.-Agitation optional (using compressed air, say). Rinsings Chromium plating Rinsings Drying The cracking-nickel-plating baths used in the conditions specified in the two foregoing examples give, for plating times of less than two minutes, a coating of brightness such as to insure the subsequent obtainment of very bright chromium plating.

In certain special cases, however, it may be necessary to extend the duration of the cracking-nickel-plating operation for a considerably greater time. This may be the case, for instance, when the parts to be protected have heavily recessed portions, since the thickness will increase less rapidly in such places than on the raised portions. It may also happen that certain installations, by their very design, will not permit of reducing the duration of the cracking-niekel-plating to 2 minutes or less.

It is then necessary to add to the bath one or more suitable brightening substances. Such brightening substances Inust not appreciably affect the potential of the cracking-nickel deposit, so that the electrochemical protective process may be maintained as described above. In addition, such additives must not reduce the propensity for cracking of the ultimate coat of nickel which generates the microcracking in the subsequent chomium plating.

By way of brightening agents it is possible to utilize the sulfonates, the sulfonamides and the sulfonimides of aromatic compounds. More specifically, the sodium salt of sulfonimide ortho-benzoyl acid (saccharin) can be added in proportions ranging from 0.5 to 3 g./l., the optimum being around 1 g./liter, though this is by no means critical.

If added to the cracking-nickel-plating baths, butyne 1-4 diol will enable very bright deposits of cracking nickel to be obtained, regardless of the thickness of the deposits. This compound is added to the bath in the proportion of 0.1 to 3 g./ liter.

Other brightening agents which will enable particularly bright cracking-nickel deposits to be obtained, regardless of their thickness, are the amino derivatives of heterocyclic compounds such as Z-amino-thiazole.

Lastly, when cobalt is added to the cracking nickel baths, it also enables the brightness of the deposit to be enhanced for concentrations ranging from 1 to 30 g./liter.

The other products added when. necessary to these ingredients were intended to reinforce the bright or glossy appearance of the deposit.

The thickness of the cracking-nickel deposit can be considerably increased by the aid of the brightening agents utilized in accordance with the invention. This enables the method to be used in existing installations with minimum or no modification.

Indeed, this leads to the possibility of a simplified application: for when the electrolysis baths permit of 0btaining bright and cracking deposits of a thickness ap" preciably greater than a few microns, for instance 10 to 20 microns, then in accordance with the present invention these deposits can be substituted for the terminal protective nickel coating instead of being superimposed thereon.

In the case of certain brightof semibright nickel-plating baths in whichthe additives may result in a degree of surface passiveness, perfect adhesion of the cracking-nickel deposit is insured by initially depassivating the bright or semibright nickel deposit. This is achieved by means of the following operations:

Careful rinsing of the bright or semibright nickel deosit; p Treatment in an alkali cyanide-base depassivation bath, for which the following formulation represents a nonlimitative example:

Caustic soda g./1 45 Anhydrous sodium carbonate g./l 45 Sodium cyanide g./l 20 Temperature: ambient.

Current density a./dm. 5 to Duration of cathode pass sec 20 to 30 Careful rinsing before insertion into the bath, thereby insuring a deposit of cracking nickel.

In cases where the initial nickel deposit is strongly passivated, provision may be made for an anode pass lasting approximately 5 seconds prior to the cathode pass.

In Brown, U.S. 3,471,271, there is described a method of electrodeposition of chromium consisting of depositing three successive layers including an underlying layer of nickel, an intermediate strike (or crackling) nickel layer and a top chromium layer. However, in order to enable the deposition of the strike or crackling layer, the method of U.S. 3,471,271 consists of using a nickel chloride bath containing dissolved therein a substantial quantity of at least one amino acid, such as EDTA, EDDA, HEDDA, DTPA.

In the aforementioned U.S. Pat. 3,388,049, a strike layer was recommended using one of the substances: nickel chloride, nickel sulfamate or nickel fluoborate with which is mixed a substance chosen from the group of acetic acid, ammonium acetate, nickel acetate and mixtures thereof.

According to the present invention, in a bath containing exclusively nickel chloride (sulfamate or fluoborate) as the substance giving rise to the formation of the nickel metal layer is added a product in addition to or a substitute for the acetic acid and nickel acetate, selected from the group consisting of gluoonic acid, nickel gluconate, tartaric acid, nickel tartrate, formic acid, nickel formate, malic acid, nickel malate, lactic acid, nickel lactate, citric acid, nickel citrate, succinic acid, nickel succinate and mixtures thereof.

The concentration of said basic products, that is, nickel chloride on the one hand, nickel acetate or the substitution products therefor on the other hand, remain similar to those described in the above said Pat. 3,388,049. The additives used for increasing the brightness as well as the conditions of operation likewise remain unchanged.

The baths incorporating acetic, gluconic, formic and succinic acid, associated with their nickel salts, in combination with nickel chloride, permit the production of the desired microcracked chromium deposits throughout the range of current densities providing bright ornamental chromium deposits. They are therefore more particularly suited for carrying out this invention.

However, the other acids or salts employed, listed above, give very satisfactory results when the shape of the parts to be electroplated is not likely to lead to abnormally high difierences in current densities.

In this case, nickel sulfamate or fluoborate may also be substituted for nickel chloride, the latter being preferred however.

By means of the bath according to this invention, there is obtained a crackling layer due to the deposit, inside the layer of nickel ions, nickel hydrate ions whose presence creates internal stresses in the hard nickel layer. The quantity of hydrate deposited must remain extremely small, i.e. perceptible traces that are not assessable. To this end, it is necessary for the pH of the solution in the layer directly in contact with the electrode (the so-called pellicular layer) to remain at a very precise value, i.e. 6.8 :tseveral hundredths.

i a H The above is possible with the mixture according to the invention which is a buffering mixture containing organical acids and nickel salts thereof.

Although this invention is not limited by any theory of operation, it appears that this phenomenon can be explained as follows: There is produced at the cathode in a known manner the electrolysis of water providing H+ and (OH)- ions at the same time as the electrolysis of the Cl Ni provides cations Ni+ and anions Cl-. At the cathode there is therefore'a disassociation of H and Ni. Certain Ni ions combine with the (OH) ions to provide nickel hydrates Ni+ (OH)" This disassociation produces a remarkable, advantageous result for a pH value of 6.8: several hundredths at the cathodic film. The pH value at this location is clearly greater than that of the bath per se which may vary, as it is stated, from 2-6 according to the products present and their concentrations.

When the pH of the cathodic film increases towards the value 7, the basic character increasing, the amount of (OH) ions provided increases and therefore the proportion of the nickel hydrate relative to the total Ni also in creases. In this case, as it has been stated, the nickel layer contains too large a quantity of hydrates. For even greater values of pH in the pellicular layer, say 7.5, a deposit is obtained without adhesion, which is pulverulent and has an inadequate appearance.

If, on the contrary, the pH of the cathodic film decreases, the amount of (OH) ions provided also diminishes and the probability of hydrate formation becomes practically zero: the H and OH ions recombine immediately.

Thus, it is necessary to operate with a cathode pH between very narrow limits which are practically immeasurable and which prescribes the choice of the above-mentioned buffering mixtures.

Moreover, it is well known that nickel sulfate provides a softer nickel layer than the nickel chloride. In this respect, see the publication of International Nickel Corp., 1944. Nickel chloride provides a nickel layer in which the creation of internal stresses causing the crackling are developed at the maximum, whereas the presence of softer nickel zones, even in very reduced amounts, due to the presence of nickel sulfate in the bath has the effect of dissipating the internal stresses and therefore preventing the formation of the strike or crackling layer.

In all the examples described in the Brown patent, U.S. 3,471,271, nickel sulfate was employed in combination with nickel chloride, so that no crackling layer could be directly obtained, even by substituting the citric acid for amino acid. The formation of small cracks in U.S. 3,471,- 271 is caused by the action of the amino acids on the deposited nickel layer.

This explains why in the experiments described in Brown, where the presence of nickel sulfate opposed the formation of a hard nickel layer, no formation of microcracks occurred even for the pH value of 6-8 for the cathodic film. Thus, according to Brown, the presence of amino acid is necessary to obtain the crackling effect. According to the instant invention, the presence ofamino acids is not necessary since nickel chloride (sulfamate, fiuoroborate) is used to the exclusionof all nickel sulfate, the product is chosen amongst the above enumerated substances and the operation is carried out so as to maintain a pH value in the cathode film strictly in the vicinity of 6.8.

What is claimed is: Y

1. A method of producing a corrosion resistant microcracked coating of chromium on a substrate which comprises first electro-depositing nickel on said substrate from an aqueous acidic bath containing (a) g./l. to a saturation amount of a nickel salt selected from the group consisting of nickel chloride, nickel sulfamate and nickel fluoborate, and (b) at least one compound selected from the group consisting of gluconic acid, tartaric acid, formic acid, malic acid, lactic acid, citric acid, succinic acid and the nickel salts of said acids in an amount to provide the nickel deposited with a high internal stress level, said bath containing no nickel sulphate, the pH in the pellicular zone being about 6.8, said stress level being sufiicient to produce micro-cracks therein during the deposition of the chromium layer which in turn produces a micro-crack pattern in the chromium layer and thereafter electrodepositing chromium on said nickel.

2. The process as set forth in claim 1 wherein (a) the salt is nickel chloride and (b) the compound is selected from the group consisting of formic acid, gluconic acid, succinic acid and the nickel salts thereof.

3. The process as set forth in claim 1 wherein the bath includes at least one brightening agent.

References Cited UNITED STATES PATENTS 11/1871 Keith 204-49 2/1971 DuRose et al 20449 X 10/1969 Brown et a1 204-49 X FOREIGN PATENTS 6/ 1961 Great Britain 20449 US. Cl. X.R. 

